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Abstract
Laser-induced breakdown spectroscopy is a promising method for rapidly measuring hydrogen and its isotopes, critical to a wide range of disciplines (e.g. nuclear energy, hydrogen storage). However, line broadening can hinder the ability to detect finely spaced isotopic shifts. Here, the effects of varying plasma generation conditions (nanosecond versus femtosecond laser ablation) and ambient environments (argon versus helium gas) on spectral features generated from Zircaloy-4 targets with varying hydrogen isotopic compositions were studied. Time-resolved 2D spectral imaging was employed to detail the spatial distribution of species throughout plasma evolution. Results highlight that hydrogen and deuterium isotopic shifts can be measured with minimal spectral broadening in a ∼ 10 Torr helium gas environment using ultrafast laser-produced plasmas.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Quantitative analysis of hydrogen (H) and its isotopes is critical to numerous fields, including nuclear forensics and safeguards, hydrogen storage, characterization of hydride formation and hydrogen embrittlement, tritium retention in plasma-facing components of fusion reactors, biogeochemistry, and more [1,2]. Hydrogen in materials can either be desirable, for example in hydrogen storage applications, or undesirable in the case of H enhanced material degradation. In either case, the concentration and distribution of H and its isotopes at the micro- and macro-scales are critical to understanding underlying mechanisms of H retention and trapping in materials [3]. For nuclear fuel cladding, it is essential to monitor the amount of H pick-up during waterside corrosion, and the precipitation of brittle hydride phases that can lead to component cracking [4,5]. In addition, the detection of tritium (T or 3H) produced and retained in the getter component of tritium producing burnable absorber rods (TPBARs) is essential to sustained T production for the strategic stockpile [6].
Several methods have been investigated for the analysis of H isotopes in alloys, including isotope ratio mass spectrometry (IRMS), nanoscale secondary ion mass spectrometry (NanoSIMS), time of flight SIMS (ToF-SIMS), atom probe tomography (APT), and nuclear reaction analysis [3,7–10]. Radiation detectors can also be used for measurement of T, but are not practical for analytical or standoff measurements due to the very short mean free path (∼ several mm) of the tritium decay soft beta emission in air. For any technique, distinguishing between solute H and that which was introduced into the material during sample handling or analysis remains a challenge. Also, many of these techniques are not rapid, with limited throughput due to time-consuming sample preparation and data collection processes. Laser ablation coupled with optical emission spectroscopy (LA-OES), often referred to as laser-induced breakdown spectroscopy (LIBS), is a possible rapid, non-contact method for the detection and quantitative analysis of various isotopes including H [11–18]. With no sample preparation requirement, a relatively simple experimental set-up, and the ability to detect all elements in the periodic table in the matter of seconds, the LA-OES technique can be employed for measurement of H isotopes in virtually any target of interest (gas, liquid, or solid) [18–21]. However, quantifying the concentration of H and its isotopes (i.e. D and T) is challenging via LIBS due to spectral line broadening and H contamination issues [22]. Line broadening can hinder the ability to detect H and its isotopes if linewidth is greater than the isotopic shift [11]. Line broadening becomes increasingly important for finely spaced isotopic shifts, such as those for H and D (0.18 nm), and H and T (0.24 nm). Furthermore, spatial segregation of emission from plume species, the presence of H and D fine structure components, as well as H present as a minor impurity in the target and ambient environment further complicate the analysis of H isotopes via LIBS [23].
The evolutionary history of all species in a laser produced plasma (LPP) depends strongly on the initial physical conditions of the plasma as well as the pressure and chemistry of the ambient [11,24,25]. Since the nature of laser irradiation, in particular the laser pulse duration (i.e. pulse width), affects plasma properties, the linewidth and shape of an optical transition are also influenced by plasma generation conditions. Previous studies showed that plasmas produced by ultrashort laser pulses provide certain advantages over traditional ns laser pulsing such as reduced continuum and matrix effects, and generation of a primarily atomic plume with low degree of ionization and relatively low temperature [11,26]. In this article we investigate the impact of the nature of ambient gas as well as plasma generation condition (i.e., nanosecond vs femtosecond LA) on the line broadening and the ability to detect H isotopes in an industrially relevant alloy, Zircaloy-4. Zircaloy-4 is the focus of the present study since this alloy is commonly used in applications in which the analysis of H is essential (i.e., in nuclear reactors as fuel cladding and as a T absorbing material [6]). Our results show that ultrafast LA coupled with emission spectroscopy in a moderate pressure helium (He) gas environment is well-suited for distinguishing between Zircaloy-4 samples with different H concentrations. The differences in spectral features of plasmas generated from Zircaloy-4 targets with varying isotopic compositions (i.e. D-loaded, H-loaded, and as-received) were studied. Using conditions well-suited for the analysis of H isotopes, we develop a linear calibration curve for determining H concentration in Zircaloy-4 via LIBS.
2. Experimental methods
A schematic of the experimental set-up is summarized in Fig. 1. Zircaloy-4 target materials were mounted in a cubic vacuum chamber (0.004 m3) for analysis. The vacuum chamber was placed on an x-y-z translator to easily move between targets, and to prevent drilling. Optical windows for laser entrance and light collection were included in the chamber design. A pressure gauge, vacuum pump, and a gas line were also attached to the chamber to control ambient gas pressure and chemistry. High purity Ar and He gases (∼99.99%) were used for analyses.
Fig. 1. Schematic of experimental set-ups used for ns and fs laser ablation and optical emission spectroscopy. Acronyms used above are: FM – folding mirror, L – lens, DP – dove prism, ICCD – intensified charged coupled device, WP – wave plate, C- cube polarizer, BD – beam dump, TFP – thin film polarizer, ns – nanosecond, fs - femtosecond.
Two laser irradiation conditions (ns and fs LA) were used for producing plasmas from Zircaloy-4 targets. A Nd:YAG laser (Continuum Surelite) with 1064 nm wavelength, ∼6 ns full-width half-maximum (FWHM), with a laser pulse energy of ∼45 mJ, and spot size of ∼ 500 µm was used for ns LA. For fs LA, a Ti:Sapphire laser (Coherent Astrella) with ∼800 nm wavelength, ∼35 fs FWHM, with a laser pulse energy of ∼5.0 mJ, and spot size of ∼200 µm was used. The laser fluence for generating plasmas was ∼ 10 J/cm2. Several cleaning shots were performed before data collection for removing oxide and surface contaminants.
Two-dimensional (2D) spectral images were collected to track the spatial distribution of H isotopes and other species within the plume. For collecting 2D spectral images, an optical system consisting of two plano-convex lenses was used for imaging the plasma plume onto the slit of the Acton Spectrapro 2500i spectrograph. The LIBS plume was imaged such that the plume expansion direction was parallel to the slit height with the help of a Dove prism [27]. The spectrograph was coupled to an intensified charged coupled device (ICCD, PiMAX4) for recording time-resolved 2D spectral images. The spectrograph and ICCD were positioned orthogonal to the plasma expansion direction. The spectrograph provided a spectral resolution of ∼ 0.016 nm, measured using a 632 nm He–Ne laser, using 2400 grooves per mm grating, with a line shape closer to Lorentzian. The emission from the plasma was also collected and analyzed in a spatially integrated manner. For these measurements, a lens was used to focus the emission to a 400 mm multimode fiber which was coupled to a 0.5 m spectrograph (Princeton Instruments Isoplane) and an ICCD (PiMAX4). Both spectroscopic systems were synchronized with the laser pulsing via a timing generator. Acquisition gate delay and integration time were varied to allow for time-resolved measurements.
3. Results
3.1 Zircaloy-4 emission spectra
The spectral linewidth relative to isotopic shift is critical for using optical spectroscopic tools for isotopic analysis [11]. The spectral linewidths and line shapes as well as emission intensity of various transitions in a LPP strongly depend on plasma generation conditions, pressure and chemistry of the ambient. Typically, the measured linewidth approaches the instrumental linewidth within a short time after the plasma onset, provided experiments are performed in low-pressure or vacuum environments where plasma free expansion reduces collisional effects as well as number density. However, the signal-to-noise and signal-to-background ratios (SNR and SBR, respectively) of emission intensity are also reduced in vacuum because of the lack of collisional excitation. Instead, the presence of an ambient gas provides collisional excitation, but also leads to line broadening due to increased temperature and density [28]. Thus, we performed a parametric study involving ambient gas pressure which showed moderate pressure levels (1-30 Torr) provide excellent SBR with reduced line broadening. Hence, a pressure of ∼ 10 Torr was selected for the present study. Previous reports have shown that moderate ambient pressure levels provide the highest SNR and SBR [28,29]. The presence of reacting species in the environment such as oxygen or air can lead to changes in the chemical composition of the plume through plasma chemistry, and reduced atomic emission due to its consumption in the formation of oxide molecules [29]. Given Zr readily reacts with oxygen, forming ZrO molecules [30], we used inert gas (Ar and He) to preclude plasma chemistry.
Traditionally, LIBS experiments are performed in a space and time-integrated manner. However, the spatial segregation between the emission from H isotopes and other emitting species in the plume (e.g., Zr) can be expected considering the high excitation energy levels of H lines (> 12 eV). So, we performed spectral analysis using a spatially and temporally resolved method. 2D spectral images were recorded for the spectral range of 653.3–658.7 nm which contains 2H (D) and 1H transitions at 656.10 nm and 656.28 nm, respectively. An example of a 2D-spectral image is given in Fig. 2(a) which shows the changes in emission intensity with distance from the target for a fs LPP at a gate delay of 1 µs, and gate width of 2 µs. Prominent Zr and H lines for the wavelength range of 653.31–658.65 nm are labeled in Fig. 2(b), including: Zr I at 655.05 nm and 656.95 nm, Zr II at 657.87 nm, 2H (D) at 656.10 nm, and 1H at 656.28 nm. The Hα and Dα Balmer emission lines contain several fine and hyperfine structures that span ∼ 20 pm [31].
Fig. 2. 2D spectral features from deuterium-loaded Zircaloy-4 sample. Femtosecond pulses were used for producing a plasma in 10 Torr Ar. The gate delay and width used were 1 µs / 2 µs.
3.2 Time resolved emission spectra in Ar and He
Time resolved spectral images were collected for both ns and fs LPPs in 10 Torr Ar, with results given in Fig. 3 for three targets with varying H or D concentrations viz. D-loaded Zircaloy-4 (∼ 1.3 wt. % or ∼ 38 at. % D), H-loaded Zircaloy-4 (∼ 0.45 wt. % or ∼ 29 at. % H), and an as-received Zircaloy-4 target which contains 25 ppm H (with no H or D loading). Spectra collected from D-loaded and H-loaded samples show isotopic shifts, and analysis of the as-received Zircaloy-4 sample provides trace H from the target or impurity H in the environment. Time resolved 2D spectral images were taken from 1-26 µs for ns LPPs, and 200 ns – 1 µs for fs LPPs and representative images are only given (Figs. 3 and 4). The gate widths (i.e., integration times) selected for analyses vary between ns and fs LPPs due to differences in emission signal strength over plasma lifetimes. This is partly due to differences in laser energies used (45 mJ for ns LA and 5 mJ for fs LA) as well as increased persistence of ns LPP due to laser-plasma coupling. A progressive gate delay was used for studying ns LPPs for accounting weak emission features at later times of plasma evolution.
Fig. 3. Spectral features of ns and fs laser produced plasmas in 10 Torr Ar: Ns LPP spectra were taken at (a) 1 µs / 1 µs and (b) 26 µs/ 2 µs gate delay/width. Fs LPP spectra were taken at (c) 200 ns/ 2 µs, and (d) 1 µs/ 2 µs. Spectral features given are all taken from a distance of 3 mm from the target. Three different samples were analyzed: D-loaded (∼ 0.6 D/Zr), H-loaded (∼0.4 H/Zr), and as-received Zircaloy-4 (where H is only present as an impurity ∼ 25 ppm). The target material corresponding to the 2D spectral image is noted in the upper left, as follows: D (D-loaded), H (H-loaded), AR (as-received).
The time and space resolved spectral features recorded in an Ar ambient from ns and fs LPP showed significant differences. Notable among them are: (1) higher emission persistence in ns LPP, (2) reduced spectral broadening in fs LPP even at very early times, and (3) spatial segregation between H and Zr emission. An inert Ar cover gas is typically preferred for LIBS studies as it provides good SNR and SBR along with increased emission persistence due to higher plasma temperatures compared to other ambients, such as air [32]. Although the spectra given in Fig. 3 show clear isotopic shifts between H and D for both ns and fs LPPs, a significant broadening is apparent in the case of ns LPP at early times, which is due to the Stark effect [33–35]. However, at late times of ns plasma evolution, the broadening is reduced (Fig. 3(b)). The fs LPPs were studied over a different time window due to its reduced persistence in comparison to ns LPPs [11]. For fs LPPs, spectral features demonstrate significantly reduced line broadening of D and H lines even at early times of its evolution ∼ 200 ns after plasma onset (Fig. 3(c)). The increased persistence in the case of ns LPPs is attributed to the efficient laser-plasma heating. Since laser-plasma heating is negligible in ultrafast LPPs, they generate relatively lower-temperature conditions even at early times, and hence its persistence is reduced compared to ns LPPs [11]. Hence, the spectral broadening is found to be less in fs LPP throughout its entire lifecycle. The spatial segregation of emission from H isotopes (i.e., H, D) compared to Zr emission lines is also apparent in both ns and fs LPPs, where the emission from H is greater closer to the target compared to Zr. This segregation is related to the requirement of higher temperature conditions for H I emission due to its high upper energy level (≥ 12 eV) compared to Zr I (upper energy levels ∼ 2.5-4 eV) [36].
The spectral features recorded in Ar ambient showed differences in emission intensity of H contaminants. The H peak intensity at 656.28 nm is expected to be similar for both D-loaded and as-received samples, yet we find the intensities are dissimilar for both ns and fs LPP. This difference could be related to the existence of inhomogeneities in the plume, as well as the hotter conditions at early times when H emission in dominant.
Previous studies highlighted that He ambient generates cooler conditions in the plasma due to its high thermal conductivity compared to Ar [32]. Thus, we evaluated the line broadening effects in a He ambient under similar conditions, with results summarized in Fig. 4 for both ns and fs plasma generation conditions.
Fig. 4. Spectral features of ns and fs laser produced plasmas in 10 Torr He: Ns LPP spectra taken at 3 mm from the target at (a) 1 µs / 1 µs gate delay/ width, and (b) 26 µs/ 2 µs, and fs LPP spectra at (c) 200 ns/ 2 µs, and (d) 1 us/ 2 µs. 2D spectral images below each were taken at the same gate delay/width timing given in the spectrum above. Details regarding target materials are provided in the Fig. 3 caption.
3.3 Linewidth
Linewidth as a function of time for the H I line at 656.28 nm was determined by fitting spectra for the H-loaded sample with a Voigt model to determine the FWHM, with results given in Fig. 5 for ns/fs LPP in both Ar and He ambients. Line broadening for D and H in D-loaded and as-received Zircaloy-4 samples exhibited similar trends, and are thus not reported here. Figure 5 illustrates that the spectral profiles are broadened in an Ar ambient irrespective of plasma generation conditions compared to He. The Ar ambient provides better confinement compared to He due to heavier atomic mass (mAr = 40 amu vs mHe = 4 amu) and hence denser plasma plumes. Previous studies also highlighted that the presence of Ar ambient leads to hotter plasma conditions compared to He background due to the presence of larger number of metastable levels which effectively transfer the energies to analytes during collisional energy transfer [15]. The existence of hotter conditions leads to increased line broadening due to the Doppler effect which scales with the square root of temperature [37]. These two factors (confinement and hotter conditions) will lead to greater broadening in Ar ambient compared to He. The lighter He gas with higher thermal conductivity helps to cool the plasma more efficiently, and thus reduce line broadening.
Fig. 5. Time evolution of H I (656.28 nm) linewidths for ns and fs LPPs in Ar and He ambient at 10 Torr. Linewidths were determined from the full width half maximum (FWHM) of the Voigt fit of H I lines from the time and spatially resolved spectra ∼ 3 mm from the target. The Zircaloy-4 target analyzed was the H-loaded sample (∼0.45 wt. % H). Experimentally measured line widths are fit with exponential decay curves as a guide for the eye. The dashed line represents the instrumental linewidth of the emission spectroscopy system.
3.4 Comparison of hydrogen emission intensity with a known concentration
Applying experimental conditions well suited for H isotopic analysis, we collected spectra from Zircaloy-4 target materials with H-loadings ranging from ∼0.2 to 1.1 wt. % H via fs LIBS in a 10 Torr He ambient. A calibration curve (Fig. 6) was then developed by plotting the area under the H I (656.28 nm) curve versus H concentration in wt. %. The close agreement of the H I emission intensity measured via LA-OES with the regression line demonstrates that fs LA in a 10 Torr He ambient can be used to accurately measure H/Zr concentrations. LA-OES using ultrafast (i.e. fs) laser pulsing is excellent for several practical applications in which high spatial resolution and depth profiling are needed (e.g. post-irradiation examination of fuel plates or TPBARs). Ultrafast laser pulsing also results in minimal heat-affected zones, ideal for materials characterization efforts. Using fs LA-OES, H concentration can rapidly be obtained and compared to calculated (expected) values to estimate H pick-up during waterside corrosion of fuel plates, or 3H retention in Zircaloy-4 getters in TPBARs.
Fig. 6. The H I emission intensity for various H-loaded Zircaloy-4 targets via fs LIBS in 10 Torr He gas. H I emission intensity was calculated by fitting the H I (656.28 nm) line with a Voigt fit and taking the area under the curve. Emission intensities were calculated from time-resolved, spatially integrated Zircaloy-4 spectra from target materials with different H loadings, reported in wt. % on the x-axis. The error associated with calculated areas are smaller than the plotted points, hence are not visible above. A constant gate width of 2 µs was used for measurement, and spectra used for calibration were the average of two laser shots.
4. Discussion
Several key aspects can be derived by comparing Ar vs He ambients (Fig. 3 and Fig. 4), most prominently: (1) reduced spectral broadening in He (for both ns and fs LPPs); (2) higher emission intensity for H, D and Zr emission lines in Ar (but reduced SBR due to higher background), (3) stronger confinement in Ar, and (4) good correlation between H trace/impurity emission from as-received and D-loaded sample in He.
For all times and environments, the linewidth of H I in He for fs LA is narrower in comparison to ns LA, although at later times the difference is small (∼ 3%), and within the experimental error. While FWHM are comparable at later times for ns and fs plasmas, they both reach a minimum value although the measured values are ≥ 2× higher than the instrumental linewidth (∼ 0.016 nm). Typically, the Stark effect is the prominent line broadening mechanism in LPPs in the early times of its evolution and the it varies quadratically with electric field generated by charged particles in the plasma. However, the H atoms have a linear Stark effect [38], hence significant Stark broadening can be expected even at later times of plasma evolution. Being the lightest element, the Doppler effect is also predominant in H line profiles and becomes a competing component for line broadening with Stark effect at even at early times. Hence, the line broadening observed in the present study could be due to the combined effect of both Doppler and Stark effects. The Hα and Dα lines contain fine and hyperfine structure components which span over ∼ 20 pm. The combination of these effects explain why the instrumental limit is not reached, even at later times in plasma evolution. Hence the given spectral profiles represent a convolution of various broadening effects and higher resolution spectroscopy tools such as laser absorption spectroscopy (LAS) and laser-induced fluorescence (LIF) are essential to separate individual line broadening contributions [11,39].
The temperature and electron density were measured from recorded spectral features for evaluating their contribution in spectral broadening mechanisms in H emission. Electron density (Ne) was calculated from the Lorentzian width of the H I line at 656.28 nm [35,40]. The measured electron density in Ar ambient for ns LPP is ∼ 4.0 ${\times} $ 1016 cm-3 at 1 µs and ∼ 6.5 ${\times} $ 1015 cm-3 at 200 ns for fs LPP. In He gas, the respective Ne values at early times for ns and fs plasmas are ∼4.6 $\; \times $ 1015 cm-3 (1 µs) and ∼ 2.6 $\; \times $ 1014 cm-3 (200 ns). These measured values suggest that the Stark effect is less in fs LPP and in He ambient at earlier times of plasma evolution. The H line broadening due to the Stark effect approaches the instrumental line broadening when the electron density is ∼6 ${\times} $ 1014 cm-3.
Temperature measurement are made using Zr I lines assuming plasma is in local thermal equilibrium (LTE) and using spectroscopic constants available in the literature [36]. The estimated temperature values are ∼7500 K and ∼5500 K for ns LPP in Ar and He, respectively at ∼11 µs after the plasma onset. In contrast, fs LPPs generate cooler conditions even at very early times. The measured temperatures showed ∼5700 K and ∼4200 K in Ar and He at 600 ns after the plasma onset. Assuming the various temperatures of the plasma are similar (i.e. excitation, kinetic etc.), we can estimate the Doppler broadening contribution in Ar and He ambient. The contribution of Doppler effect is ∼41 pm and ∼30 pm at 7500 K and 4200 K respectively and the Doppler broadening approaches instrumental broadening when the plasma temperature is ∼ 1300 K. Typically, the Doppler contribution is negligible in emission-based measurements compared to instrumental. However, the Doppler effect is found to be a significant line broadening mechanism for H lines in LPPs. For ns and fs LPPs in He, we experimentally measure approximately similar H linewidths at later times in plasma evolution. Barring instrumental broadening, which is consistent for both ns and fs plasmas, a balancing act between Doppler and Stark effects gives similar linewidths at later times in He for both ns and fs LPP, although at different times. 2D spectral images clearly show the distribution of D, H, and Zr emission intensities are spatially segregated within the plumes for all conditions. Zr emission intensity appears farther from the target in comparison to D and H in both Ar and He. However, in He all species exist farther from the target (versus Ar) due to reduced confinement by the lighter cover gas. A very weak broadband emission features are also visible in the fs spectral images, which may be due to nanoparticle emission [27]. A better correlation for H emission intensity from as-received and D-loaded samples is also seen in He in comparison to in an Ar environment. From bulk chemical analysis of the as-received Zircaloy-4, H content is estimated at 25 ppm, which should also be present in D and H-loaded samples. The good agreement between the H I line shape and intensity in D-loaded and as-received spectra suggests ∼25 ppm H trace can be easily detected in 10 Torr He. Prior work showed that the H detection limit using LIBS is ∼10 ppm [13].
Our results highlight that the emission persistence is approximately similar for both He and Ar. Although it is anticipated that fs LPP provides narrower line profiles even at early times of plasma evolution, it is intriguing that the moderate pressure He ambient provides similar levels of plasma persistence in comparison with Ar and is better suited for H isotopic analysis. Prior work suggests energy transfer from metastable excited He atoms by collision processes is responsible for strong emission signal with low background, and narrow width [13]. In addition, plasmas generated in He are cooler (than those produced in Ar), which contributes smaller linewidths in both ns and fs LPPs.
5. Conclusion
The rapid analysis of H isotopes in Zircaloy-4 target materials with varying isotopic compositions were evaluated using LIBS. Specifically, the impact of plasma generation conditions (ns versus fs LA) and ambient gas chemistry (Ar versus He) were investigated. Time-resolved 2D spectral images were collected in order to detail the spatial and temporal behavior of species within Zircaloy-4 plasmas. We found significant spectral broadening occurs in Ar compared to He for both ns and fs cases. In addition, spatial segregation of emission between H isotopes and Zr within the plume was also observed for all conditions, where Zr emission is found farther from the target, and H and D emission is closer to the target in the hotter, denser region of the plasma. Our results demonstrate that minimal spectral broadening of D and H lines occurs in moderate pressure (10 Torr) He gas, using fs laser ablation. A linear calibration curve was developed for fs LIBS in a He ambient for measurement of H in Zircaloy-4 targets. Since ultrafast LA provides high spatial resolution, better control for depth profiling, minimal heat-affected zones, it is well-suited for analyzing H isotopes (H, D and T) in TPBAR components. In addition, this work demonstrates a promising method for the rapid detection and quantitative analysis of H and its isotopes for a wide range of safeguards and materials characterization applications.
Funding
National Nuclear Security Administration Tritium Modernization Program.
Acknowledgments
The authors thank Mr. Joshua Silverstein for providing samples for analysis and Mr. Monte Elmore for performing hydrogen loading. The authors thank Dr. Ewa Ronnebro for reviewing the manuscript, and helpful discussions related to the hydrogen loading process. All work reported in this study was performed at Pacific Northwest National Laboratory, which is operated for the U.S. DOE by Battelle Memorial Institute under Contract No. DE-AC05-76RLO1830.
Disclosures
The authors declare no conflicts of interest.
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